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NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

11 08月
作者: Vicent Peris (PTeam/OAUV) |分类:天文资料 | 转载时间:2020年08月11日
【特别声明】 本文摘自网络,原文 作者/网站: Vicent Peris (PTeam/OAUV)

By Vicent Peris (PTeam/OAUV) (由 Vicent Peris(PTeam/OAUV)提供)

Published August 8, 2020(发表日期:2020年8月8日)



Introduction(介绍)


This article describes some specific or unusual image processing techniques that I have implemented for my pictures of comet NEOWISE. You can see these pictures in PixInsight Gallery, as well as in OAUV's website. (这篇文章描述了我为我的NEOWISE彗星图片实现的一些特殊或不寻常的图像处理技术。你可以在PixInsight画廊和OAUV的网站上看到这些图片。)


The images presented here have been acquired with two different telescopes:(这里展示的图像是用两种不同的望远镜拍摄的:)


    1、A narrow field image acquired with a Planewave 20 inch f/6.7 CDK telescope and an FLI Proline PL16803M camera.(用20英寸 f/6.7 CDK望远镜和 FLI-Proline-PL16803M 相机获得的窄场图像)

    2、A wide field image acquired with a Canon 400 mm f/2.8 lens and an FLI Microline ML16200 camera.(用佳能400毫米 f/2.8 镜头和 FLI Microline ML16200 相机拍摄的宽视场图像。)


We are going to review four keypoints of the processing workflow:(我们将回顾处理流程的四个关键点:)


    1、The use of LocalNormalization to correct the shadows of blocking objects near the horizon.(使用 LocalNormalization 校正地平线附近阻塞对象的阴影。)

    2、The alignment of the wide and narrow field images using astrometric solutions.(使用天体测量方法对宽场和窄场图像进行校准。)

    3、Transferring a color calibration from a reference single frame to the master image.(将颜色校准从参考单帧传输到主图像。)

    4、Using multiscale and local contrast enhancement tools to control the low-contrast features of the comet tails.(使用多尺度和局部对比度增强工具来控制彗星尾的低对比度特征。)



Removing Blocking Objects Near the Horizon(清除地平线附近的障碍物)


Since the wide field telescope is set up in a dome, it was not possible to avoid some objetcs blocking the line of sight to the comet when it was very low above the horizon. For example, the following animation shows the structure of a roll-off building passing throughout the comet image sequence:(由于宽视场望远镜安装在一个圆顶上,当彗星在地平线上非常低的时候,它不可能避免一些障碍物阻挡它的视线。例如,以下动画显示了穿过彗星图像序列的滚落建筑物的结构:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


Since this is a thin post in the line of sight, it produces shadows that are not completely opaque, similar to a dust shadow in a flat field image. Therefore, we can correct these moving shadows in the image sequence by using LocalNormalization. We'll use a reference subframe that isn't affected by the shadow to apply LocalNormalization to the entire sequence. These are the tool seetings:(由于这是视线中的一个细柱,它产生的阴影不是完全不透明的,类似于平场图像中的灰尘阴影。因此,我们可以使用局部归一化来校正图像序列中的这些移动阴影。我们将使用不受阴影影响的引用子帧对整个序列应用 LocalNormalization 。以下是工具查看:)



NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


The reference image is the third subframe in the sequence. This is the one acquired just before the shadows appear in the image. This frame was selected because it is the unaffected one with a darkest sky—remember that this comet was very close to the Sun, which forced us to start shooting in the twilight.(参考图像是序列中的第三个子帧。这是在图像中出现阴影之前获取的图像。之所以选择这一帧,是因为它是天空最黑暗的一帧,记住这颗彗星离太阳非常近,这迫使我们在黄昏时开始拍摄)


Note that the No scale component check box is checked. Unchecking it in this image could be dangerous because there are large areas of empty sky that could originate false structures coming from the noise on the background areas. Therefore, LocalNormalization is simply adapting the local sky background level between the reference and the target images.(请注意,已选中“无比例元件”复选框。在这张图片中取消选中它可能是危险的,因为有大片的空天区域可能会产生背景噪音产生的虚假结构。因此,局部归一化只是在参考图像和目标图像之间调整局部天空背景。)


We set the Apply normalization parameter to View execution only, so the tool will only generate the XNML files. This process is applied to the image set of each filter, and then we integrate the three master images using the XNML files with ImageIntegration.(我们将 Apply normalization 参数设置为仅查看执行,因此工具将只生成 XNML 文件。该过程应用于每个过滤器的图像集,然后使用 XNML 文件和 ImageIntegration 集成三个主图像。)


In the following animation, we can see that the process removes the shadows almost completely from the images:(在下面的动画中,我们可以看到该过程几乎完全从图像中移除阴影:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)



Aligning the Wide and Narrow Field Images(对齐宽场和窄场图像)


The published wide field image is a combination of both the narrow and wide field images, since the CDK shows much more detail in the comet's head. To combine them, we should align the comet, but the master images don't have any stars, so we cannot use StarAlignment to register the 20-inch telescope image to the wider one. Below you can see the images we want to align:(公布的宽视场图像是窄场和宽视场图像的结合,因为CDK显示了彗星头部更多的细节。为了把它们结合起来,我们应该使彗星对齐,但是主图像中没有任何恒星,因此我们不能使用星光排列来将20英寸望远镜的图像与更宽的图像进行配准。下面您可以看到我们要对齐的图像:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field master image(宽视场主图像)



NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Narrow field master image(窄场主图像)


To overcome this problem, we'll perform an astrometry-based alignment. First, we select a single subframe from each image set:(为了克服这个问题,我们将进行基于天体测量的校准。首先,我们从每个图像集中选择一个子帧:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Single frame of the wide field image set(宽视场图像集的单帧)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Single frame of the narrow field image set(帧窄场图像集)


By computing an astrometric solution for both images, we can find the rotation and image scale differences between both images. Once this is done, we can rotate the narrow field master image with the Rotation tool, and resize it with the Resample tool to fit the rotation angle and the image scale of the wide field image:(通过计算两幅图像的天体测量解,我们可以找到两幅图像之间的旋转和图像比例差异。完成此操作后,我们可以使用旋转工具旋转窄场主图像,并使用重采样工具调整其大小,以适合宽视野图像的旋转角度和图像比例:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Original narrow field image.(原始窄场图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Narrow field image after rotation and rescaling(旋转和重新缩放后的窄场图像)


Now we have the comet with the same scale and rotation on both the narrow field and wide field images. Then we need to align both comet nuclei. We'll do this with DynamicAlignment, by simply selecting the comet nucleus on each image. This will align both comets:(现在我们在窄视场和宽视场图像上看到了具有相同比例和旋转角度的彗星。然后我们需要把两个彗星核对齐。我们将使用 DynamicAlignment 来实现这一点,只需在每张图像上选择彗星核。这将使两颗彗星对齐


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Narrow field image aligned to the wide field image.(窄场图像与宽视野图像对齐)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field image(广域图像)



Calibrating the Color of a Comet Image(校准彗星图像的颜色)


In this section we review the color calibration steps needed before attempting to combine both wide field and narrow field images. Our starting point are two master images, each being an integration of its respective subframes. The comet in the narrow field image has been aligned with the comet in the wide field image.(在本节中,我们将回顾在尝试合并宽视场和窄视场图像之前所需的颜色校准步骤。我们的起点是两个主图像,每一个都是其各自子帧的集成。窄场图像中的彗星已与宽视野图像中的彗星对齐。)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field master image(宽视场主图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Narrow field master image(窄场主图像)


If we want to calibrate the color of these images, we have to face a problem: the images are almost starless, since the stars are rejected by ImageIntegration, having only faint traces of the stars due to the linear displacement of the comet nucleus. Therefore, we cannot use stars to calibrate the color of these pictures.(如果我们想校准这些图像的颜色,我们就必须面对一个问题:这些图像几乎是没有星星的,因为恒星被图像整合所拒绝,由于彗星核的线性位移,只有微弱的恒星痕迹。因此,我们不能用星星来校准这些照片的颜色。)


To solve this problem we calibrate the color in a reference RGB image that is composed of a single frame for each filter. So we first compose a color image from the third frame of the R, G and B filters. Since this is a single subframe from each filter, the composed image has all of the stars.(为了解决这个问题,我们在一个参考RGB图像中校准颜色,该图像由每个滤波器的单个帧组成。因此我们首先从 R 、G 和 B 通道的第三帧合成彩色图像。因为这是来自每个滤波器的一个子帧,所以合成的图像包含了所有的恒星。)


For this methodology to work properly, we first need to align the comet in the entire data set with the CometAlignment tool. Therefore, the stars in the reference image will show a little color displacement, since the comet has moved from frame to frame. Below we show this reference image before any color adjustment:(为了使这种方法能够正常工作,我们首先需要将整个数据集中的彗星使用 CometAlignment 工具对齐。因此,参考图像中的恒星将显示出一点颜色位移,因为彗星已经从一帧移动到另一帧。下面我们将在任何颜色调整之前显示此参考图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


Now we can perform a photometry-based color calibration with the PhotometricColorCalibration tool:(现在,我们可以使用 PhotometricColorCalibration 工具执行基于光度的颜色校准)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


Here are the parameters used:(以下是使用的参数:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


Note that we are using a Sun-like star by setting the White reference parameter to "G2V Star". We chose this white reference in this case because the comet is actually illuminated by the Sun. We also set a big aperture of 12 pixels to be gentle with the possible effects of the color channel displacements in the stars.(注意,我们使用的是类太阳恒星,方法是将白色参考参数设置为“G2V星”。我们在这个例子中选择这个白色的参照物是因为彗星实际上是被太阳照亮的。我们还设置了一个12像素的大光圈,以缓和恒星中颜色通道位移的可能影响。)


Now that we have calibrated the color of the reference image, we need to transfer the color balance to the master wide field image. We'll also perform this transfer to the close-up image of the 20-inch telescope.(现在我们已经校准了参考图像的颜色,我们需要将颜色平衡转移到主广域图像。我们还将对20英寸望远镜的特写图像进行这种转换。)


The color balance transfer is performed with LinearFit. This process will adapt the signal of the master images to the reference image. It will work even if we don't have stars in the master image, since the position of the comet is the same in the reference and master images. For the wide field image, we can simply use LinearFit by setting the proper reference image and applying it to the master image:(使用 LinearFit 执行颜色平衡传输。此过程将使主图像的信号适应参考图像。即使主图像中没有恒星,它也能工作,因为参考图像和主图像中彗星的位置是相同的。对于广域图像,我们可以通过设置适当的参考图像并将其应用于主图像来使用 LinearFit :)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


This process effectively corrects the color of the wide field master image:(此过程有效地校正了广域主图像的颜色:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field master image before color correction(彩色校正前的宽视场主图像)



NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field master image after color correction(彩色校正后的宽视场主图像)



However, we need to transfer the color balance very accurately over the comet's head area because there we'll integrate the 20-inch telescope image, so both the wide and narrow field images can be exactly fitted. For this reason, we'll calculate the linear fit specifically in the small common area between the three images. This area is highlighted by the preview in the figure below:(然而,我们需要非常精确地在彗星头部区域传输颜色平衡,因为我们将在那里整合20英寸望远镜图像,因此宽场和窄场图像都可以精确地拟合。因此,我们将计算线性拟合,特别是在三幅图像之间的小公共区域。此区域由下图中的预览突出显示:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


We can clone this preview on the master images and then make these previews independent images, where we'll calculate the linear fitting parameters. So now, besides the original three images, we have three new ones:(我们可以在主图像上克隆这个预览,然后使这些预览独立于图像,在这里我们将计算线性拟合参数。所以现在,除了原来的三幅图像,我们还有三幅新的:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Preview of the color reference image. 


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Preview of the wide field master image. 


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Preview of the narrow field master image.


We'll set the identifier of the reference preview image to "ref_color", and will select it as the reference image in LinearFit:(我们将参考预览图像的标识符设置为“ref_color”,并在 LinearFit 中选择它作为参考图像:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


Now we are going to perform a linear fitting between the reference and the previews of the master images. This operation will transfer the color balance:(现在我们要在主图像的参考和预览之间执行线性拟合。此操作将转移颜色平衡:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field master image before color correction. 


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Wide field master image after color correction.


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Narrow field master image before color correction. 


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Narrow field master image after color correction.


As you probably have figured out, we are only transferring the color balance between small areas of the image, but not on the entire images. To be able to work on the entire images, we should look at the linear equations that LinearFit writes to the console; we'll apply these equations to the entire master images with PixelMath. Below we show the equations to transform the wide field master image:(正如您可能已经知道的,我们只是在图像的小区域之间传输颜色平衡,而不是在整个图像上传输。为了能够处理整个图像,我们应该查看LinearFit写入控制台的线性方程;我们将使用 PixelMath 将这些方程应用于整个主图像。下面我们展示了变换宽视场主图像的方程:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


And the corresponding PixelMath instance:(以及相应的PixelMath实例:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


Note that we'll need different equations for each master image. We just have to look for the required numbers in the console. We'll apply these equations to the entire master images. These are the master images after the PixelMath processes:(请注意,对于每个主图像,我们需要不同的方程。我们只需要在控制台中查找所需的数字。我们将把这些方程应用到整个主图像上。以下是像素数学处理后的主图像:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Color-calibrated wide field master image(彩色校准宽视场主图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Color-calibrated narrow field master image(彩色校准窄场主图像)


After these transformations, we can safely mix both images because they will have exactly the same color balance.(在这些变换之后,我们可以安全地混合两个图像,因为它们将具有完全相同的颜色平衡。)



Enhancing Details Inside the Comet(增强彗星内部的细节)


In this section we propose an easy but effective workflow to process comet images in the non-linear stage. We'll focus on the narrow field images:(在本节中,我们提出一个简单而有效的工作流程来处理彗星图像的非线性阶段。我们将关注窄场图像:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


We apply two instances of the HDRMultiscaleTransform (HDRMT) process. First with 5 layers and then with 4 layers, both processes with the Lightness mask option enabled. The result is quite aggressive (this aggressiveness will strongly depend on each image and each comet) but brings out all of the small-scale, low-contrast details around the nucleus:(我们应用了 HDRMultiscaleTransformHDRMT)进程的两个实例。首先是5层,然后是4层,这两个过程都启用了亮度遮罩选项。结果是相当具有侵略性(这种侵略性将强烈地依赖于每一张图像和每颗彗星),但却显示出了原子核周围所有小尺度、低对比度的细节:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Image before HDRMTHDRMT之前的图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

HDRMT, 5 layers.(HDRMT,5层)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Additional HDRMT, 4 layers(附加HDRMT,4层)




NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Image before HDRMTHDRMT之前的图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

HDRMT, 5 layers.(HDRMT,5层)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Additional HDRMT, 4 layers(附加HDRMT,4层)



Now, we can enhance the details of the ion tail inside the dust tail with LocalHistogramEqualization. These are the settings of the tool:(现在,我们可以用局部直方图来增强尘埃尾中离子尾的细节。以下是工具的设置:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)


In this tool, the Contrast Limit parameter is very aggressive, so we'll have better control on the process by lowering the value of the Amount parameter. LocalHistogramEqualization lets us recover the large, low-contrast features of the comet inside its tail. These are the resulting images:(在这个工具中,对比度限制参数非常具有攻击性,因此我们可以通过降低 Amount 参数的值来更好地控制过程。局部直方图质量化可以让我们恢复彗星尾部的大的、低对比度的特征。以下是生成的图像:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Before LocalHistogramEqualization(在局部直方图定性之前)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

After LocalHistogramEqualization(局部组织切片鉴定后)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Before LocalHistogramEqualization(在局部直方图定性之前)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

After LocalHistogramEqualization(局部组织切片鉴定后)


Finally, we show a mouse over comparison of the three processes applied to both images:(最后,我们将鼠标悬停比较应用于两个图像的三个过程:)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Stretched image(拉伸图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

First HDRMT(第一个HDRMT)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Second HDRMT(第二个HDRMT)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

LocalHistogramEqualization(局部直方图定性)



NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Stretched image(拉伸图像)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

First HDRMT(第一个HDRMT)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

Second HDRMT(第二个HDRMT)


NEOWISE Comet Processing Notes(NEOWISE彗星的处理说明)

LocalHistogramEqualization(局部直方图定性)




Conclusion(结论)


Comet imagery has its own unique challenges, especially if we want to perform an accurate color calibration. The workflows described in this article show a creative use of the available tools, which comes from two facts:(彗星图像有其独特的挑战,特别是如果我们想进行精确的颜色校准。本文中描述的工作流展示了对可用工具的创造性使用,这来自两个事实:)


    1、We have used LocalNormalization to correct the shadows of a line-of-sight interfering object. Thus, the connection between an observational problem and a specific tool in PixInsight lets us find its optimal software-based solution.(我们使用了局部归一化来校正视线干扰对象的阴影。因此,在PixInsight中,观察问题与特定工具之间的联系使我们能够找到基于软件的最佳解决方案。)

    2、A logical, indirect connection between multiple tools in PixInsight lets us to perform the registration and photometry-based color calibration on starless images.(PixInsight中多个工具之间的逻辑、间接连接使我们能够对无星图像执行基于光度法的颜色校准。)


It is the exclusive design of the PixInsight platform what lets us to perform such unique and complex workflows, adapting them to the specific needs of each picture. The imaging platform should not pose limits to the creativity of the photographer. This has always been one of the basic principles of PixInsight development.(PixInsight平台的独家设计让我们能够执行如此独特和复杂的工作流程,使其适应每幅图片的特定需求。成像平台不应限制摄影师的创造力。这一直是郫县景观开发的基本原则之一。)


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标签:彗星 NEOWISE 后期 教程 LocalHistogramEqualization LHE HDRMultiscaleTransform HDRMT PixelMath LinearFit PhotometricColorCalibration PCC DynamicAlignment StarAlignment LocalNormalization CometAlignment
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